Linear aromatic saturated polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene-2,6-naphthalene dicarboxylate (PEN) (hereinafter referred to as PET-based polyester) have excellent properties, and are widely used as fibers, films, bottles, and plastics. In the plastic art, molded products of these polyesters are utilized in automobiles, machine parts, electric and electronic materials, building materials, containers, and various industrial goods as high performance resin materials.
In recent years, it is globally recognized that used plastic products that are recovered from factory production processes and the general consumer market need to be recycled with a view to resource saving and environmental protection. The recovery and recycling of used bottles, films, etc. made of polyester have been also been positively advancing. However, there are the practical problems such as the molecular weight of crystalline polyester tends to significantly decrease because of the heat history of molding and there is a strong tendency for the crystalline polyester to have increased numbers of free carboxyl groups at the ends of the molecules, which hinders the development of technology for recycling recovered products. As used polyester that have been recovered has decreased molecular weight as compared to virgin pellets, the molecular weight of flakes (crushed matter) of, for example, a large volume of recovered PET bottles is reduced by about half. Accordingly, when the flakes are recycled as a base resin, processability is poor, the qualities of the original PET bottles cannot be reached, and only fibers that can be molded with a low molecular weight resin, and low quality sheets can be produced. The applications of recycled materials are thus limited.
In recent years, plastic foamed articles characterized in light-weight, elasticity, and processability are currently used in large numbers mainly as packages and cushioning materials. If the low molecular weight flakes of recovered PET bottles or new PET bottles can be processed to have high molecular weight, and to have improved melt viscosity capable of foaming, an enormous foamed article market will be developed.
For increasing the molecular weight, known methods include polyester solid phase polymerization, reaction of a chain extending agent (coupling agent) with polyester end groups, or addition of other resins such as elastomers to supplement mechanical properties.
As the chain extending agent (coupling agent), the use of compounds having an isocyanate group, an oxazoline group, an epoxy group, an aziridine group, or a carbodiimide has been proposed. However, such compounds have limited reactivity, heat resistance, and stability, and therefore has limited serviceability. Among these, epoxy compounds are relatively usable as compared to compounds having other groups, and Japanese Patent Laid open No. 57-161124 discloses an addition of a monoepoxy compound, Japanese Patent Laid open No. 7-166419, Japanese Patent Publication Nos. 48-25074, and 60-35944 disclose an addition of a diepoxy compound, but epoxy compounds are problematic in reaction speed, gel generation, melt viscosity, solubility, thermostability, and molded product properties.
Japanese PCT Patent Application Laid open No. 8-508776 proposes a method where PET-based polyester recovered is melted and mixed with a bifunctional epoxy resin and steric hindered hydroxyphenylalkyl phosphonate to increase the molecular weight of the polyester. In this method, although the reaction rate is relatively high, steric hindered hydroxyphenylalkyl phosphonate is expensive, which is not adequate for use in an art requiring low cost recovery and recycling expense. An alternative method is that where rubber or an elastomer is blended into the polyester, but such blends have problems in solubility, heat resistance, and elastic modulus.
Typically, the plastic for use in forming a foamed article is polystyrene or polyethylene having a high molecular weight and high melt viscosity. The PET-based polyesters have insufficient melt viscosity, i.e., flow easily upon melting, even though the molecular weight thereof is increased (IV of about 1.0, number average molecular weight of about 17,000, weight average molecular weight of about 44,000) for a bottle application, and therefore are not suitable resins for forming a foamed articles with highly foamed. Consequently there are no foamed articles made of PET-based polyesters in the world.